On the role of the distortion energy in the ab initio calculated dimerization energy of formic acid

It is shown that the use of experimental geometries in ab initio calculations of the dimerization energy of formic acid can lead to unrealistic results. This is caused by errors in the molecular energy difference between the free molecule and the distorted molecule as it occurs in the dimer. A method is suggested that overcomes this difficulty, and it leads to a value of about 3.5 kcal/mole for this molecular energy difference.